The present invention relates to a pyrogenically prepared silicon dioxide.
Pyrogenically prepared silicon dioxide or pyrogenically prepared silica is known from Ullmann""s Encyclopedia of Industrial Chemistry, 4th edition, Volume 21, page 464.
It is known to use pyrogenically prepared silicon dioxide in the preparation of glass bodies (U.S. Pat. No. 5,207,814, EP 0 586 013 B1, EP 0 705 797 A2, EP 0 131 057 B1, U.S. Pat. Nos. 4,681,615, 4,801,318). Other uses for silica are numerous and are well known in the art.
The known pyrogenically prepared silicon dioxide has the drawback of presenting an unsatisfactory viscosity behavior in the dispersion used for the preparation of glass.
Thus, an object of the present invention is to overcome the problems relating to pyrogenically prepared silicon dioxide for use in the production of glass.
The above and other objects of the present invention can be achieve by using a pyrogenically prepared silicon dioxide with the following physicochemical properties:
1. Average particle size (D50 value) D50xe2x89xa7150 nm (dynamic light scattering, 30 wt%)
2. Viscosity (5 rpm, 30 wt%)xcex7xe2x89xa6100 mxc2x7Pas
3. Thixotropy of Ti (xcex7(5 rpm))/(xcex7(50 rpm)) xe2x89xa62
4. BET surface area 30-60 m2/g
5. Compacted bulk density SD =100-160 g/L
6. Original pHxe2x89xa64.5
The pyrogenically produced silicon dioxide of the invention has deacidification index of less than 3% in a weight basis.
Measurement methods
Particle Size
Measurement method used according to the invention: Photon correlation spectroscopy (PCS) is a dynamic light scattering process which allows the detection of particles in the range of approximately 5 nm to 5 xcexcm. From the measurement results one can also calculate a particle size distribution in addition to the mean particle diameter.
Light source: 650 nm diode laser
Geometry: 180xc2x0 homodyne scattering
Sample quantity: 2 mL
Calculation of the distribution according to the Mie theory
Procedure: 2 mL of dispersion (30 mol %) are introduced into a measurement cuvette, the temperature sensor is inserted, and the measurement is started. The measurement is carried out at room temperature.
Viscosity
Measurement method for determining viscosity: A programmable rheometer is available to examine complex flow behaviors, and it is equipped with standard rotation spindles.
Shearing rate: 5-100 rpm
Measurement temperature: room temperature (23xc2x0 C.)
Dispersion concentration: 30 mol %
Procedure: 500 mL of dispersion are introduced into a 600-mL glass beaker and examined at room temperature (statistical determination of the temperature via measurement sensors) at different shearing rates.
BET: based on DIN 66131
Compacted bulk density: based on DIN ISO 787/XI K 5101/18 (not sieved)
pH: based on DIN ISO 787/IX, ASTM D 1280, JIS K 5101/24.